1. Field of the Invention
The present invention relates to a method of and an apparatus for estimating a vehicle velocity, which are suitable for use in a vehicle having drive wheels and follower wheels, and to a method of and a system for controlling brakes, wherein when a braking force to be applied to each of the brakes is estimated from a wheel slip ratio and a wheel acceleration/deceleration so as to control each brake or when braking is changed from an antilock brake system (ABS) mode to normal braking, the optimum brake pressure increasing rate can be set upon increase in the brake pressure and the braking force can be controlled based on the optimum brake pressure increasing rate, thereby making it possible to ensure a feeling of satisfactory control
2. Description of the Related Art
In a vehicle such as a motorcar, a motorcycle or the like, a so-called brake control system is used in which a speed or velocity of each wheel placed under braking is compared with a vehicle speed and controlling of brakes is effected based on the result of comparison. In the brake control system, a slip ratio is determined from the wheel velocity and the vehicle velocity. When the slip ratio reaches a target slip ratio or above, the slip ratio is reduced by decreasing brake hydraulic pressure, thereby producing the optimum braking force.
Further, a driving force control apparatus is known which controls a driving force of an engine by adjusting the ignition timing of the engine upon a vehicle rapid start or depending on a variation in a friction coefficient of a road surface, for example. Even in the case of the driving force control apparatus, the wheel velocity and the vehicle velocity are used as data.
Now, the wheel velocity, i.e., the rotational speed of each wheel can be directly detected by a sensor. It is however difficult to directly detect the vehicle velocity by a sensor. It is also next to impossible to detect the velocity of a vehicle such as a motorcycle whose weight and size are greatly restricted to accommodate the sensor therein. Accordingly, a method is normally used which estimates the vehicle velocity from the wheel velocity.
However, there is often a situation in which an estimated vehicle velocity is set to be larger or smaller than an actual vehicle velocity under the conditions in which each wheel slips against the road surface. In this case, the brake control system or the driving force control apparatus tend to effect unsuitable control.
A brake control system is known, in which a slip ratio of each wheel against a road surface is computed from the speed or velocity of a running vehicle and the rotational speed of each wheel and the optimum braking force is applied to the vehicle based on the computed slip ratio. As an example of such a system, a control logic circuit of which is shown in FIG. 1. In FIG. 1, each of an inlet valve and an outlet valve is a hydraulic control valve for controlling hydraulic pressure applied to a caliper cylinder (hereinafter called "caliper pressure") to operate a pair of calipers which hold each brake disk therebetween.
In the same drawing, each of .lambda..sub.1, .lambda..sub.2 and .lambda..sub.3 represents a slip ratio of each wheel against the road surface. They have a relationship of .lambda..sub.1 &lt;.lambda..sub.2 &lt;.lambda..sub.3. Each of .alpha..sub.1, .alpha..sub.2 and .alpha..sub.3 represents a wheel acceleration and each of -.alpha..sub.1 and -.alpha..sub.2 represents a wheel deceleration. These values have a relationship of -.alpha..sub.2 &lt;-.alpha..sub.1 &lt;0&lt;.alpha..sub.1 &lt;.alpha..sub.2 &lt;.alpha..sub.3. Now, a parameter represented by -.alpha..sub.1 and -.alpha..sub.2 is changed from "0" to "1" when each of the wheel decelerations has reached a set value (threshold value) or less. Each of parameters other than the parameter referred to above is changed from "0" to "1" when each deceleration has reached the threshold value or more. In the case of the slip ratio, on the other hand, outputs appear on signal lines or conductors set by .lambda..sub.1, .lambda..sub.2 and .lambda..sub.3 respectively when the slip ratio has reached each of given slip ratios (.lambda..sub.1, .lambda..sub.2 and .lambda..sub.3) or above (threshold value or above).
The ABS is provided with a modulator, and when the ABS is activated, control of the caliper pressure by the driver, is modified by the operation of the modulator having the aforementioned inlet valve (normally closed to a pressurized fluid source) and aforementioned outlet valve (normally open to a fluid exit), both controlled by the above-described control logic circuit. The modulator increases or decreases the caliper pressure, in response to changes in the pressurized fluid, regulated by such valves. Depending on the operation of the control logic circuit described above, three events may occur. When the inlet and outlet valves are not operated (i.e. their normal state), the modulator releases pressurized fluid through the output valve, and increases caliper pressure, to increase braking up to a predetermined maximum. If only the outlet valve is operated (i.e. both the inlet and outlet valves are closed), the modulator remains in a constant state and likewise, the caliper pressure is kept constant. If both the inlet and outlet valves are operated (i.e. opened and closed respectively), then in response to the increased pressurized fluid, the modulator effects a decrease in caliper pressure to reduce braking. The above is summarized in FIG. 2.
Thus it is seen that the brake control is effected by setting a threshold value for each of the slip ratio and the acceleration/deceleration, and determining whether the actual state of each wheel (i.e. and ), is at their respective threshold value or above (less). It is thus necessary to set processing times as short as possible to improve the operating speed of an actuator which executes the process referred to above. However, there are limitations on the operating speed of the actuator, and an improvement is actually difficult to achieve.
In another prior art system, the modulator comprises an input hydraulic chamber which communicates with a master cylinder, for converting a brake operating instruction generated by the operation of a lever by a driver or the depression of a pedal by the driver into hydraulic pressure or power, an output hydraulic chamber which communicates with a caliper cylinder, for applying a braking force to a brake disk of each wheel (hereinafter called as "caliper force"), a cut valve for causing the input hydraulic chamber to communicate with the output hydraulic chamber and for cutting off the communication between the input and output hydraulic chambers, an expander piston disposed on the output hydraulic chamber side for closing the cut valve upon antilock braking and for increasing the volume of the output hydraulic chamber so as to reduce the hydraulic pressure or power, and a crank member held in abutment against the expander piston and rotatable by a rotative drive source.
In the modulator, the caliper pressure is reduced by displacing the expander piston so as to increase the volume of the output hydraulic chamber to avoid a locked state of each wheel upon braking. When the risk of the locked state is avoided, the expander piston is displaced to open the cut valve, thereby effecting normal braking.
In the prior art, however, when the braking is changed from the antilock braking to the normal braking, caliper pressure P.sub.c is abruptly raised toward master pressure P.sub.m developed in the master cylinder at the maximum pressure increasing rate as indicated by the broken line defined between Q and R in FIG. 3.
When a vehicle travels from a road surface having a low friction coefficient (hereinafter called a "low .mu. road") with respect to each wheel to a road surface having a high friction coefficient (hereinafter called a "high .mu. road") with respect to each wheel while the antilock control is being effected during the braking of the vehicle, the front wheel first comes across to the high .mu. road. Thus, a gripping force of the front wheel is raised so that the slip ratio is reduced, thereby enabling control for increasing the brake pressure. However, the rear wheel is still placed on the low .mu. road. Therefore, when the caliper pressure P.sub.c applied to the front wheel is simply raised at the maximum pressure increasing rate, the braking forces of the front and rear wheels against the road greatly differ from each other. This tends to hurt the control feeling. It is thus preferable to maintain the pressure increasing rate at a given value until the rear wheel reaches the high .mu. road.
A modulator provided with a double-structure type cut valve having a dual orifice defined therein is therefore known as has been disclosed in Japanese Patent Application Laid-Open Publication No. 49-15874 (which corresponds to U.S. Pat. No. 3,836,207). However, this modulator is also actuated by the pressure difference developed in hydraulic pressures, between the input hydraulic chamber and the output hydraulic chamber. Therefore, the pressure increasing rate is restricted and hence various pressure increasing rates suitable for the conditions of the road surface or the state of braking cannot be realized.